refactor: move compiled kernel .so files into extension/lib

- CUDAExtension module names changed from astrai.extension.<name> to astrai.extension.lib.<name>
- Compiled .so files now land in astrai/extension/lib/ instead of alongside Python source
- loader.py imports from .lib.<name> subpackage
- Add astrai/extension/lib/__init__.py to make lib a proper package
- Separates compiled artifacts from Python source for cleaner directory structure

astrai/extension/loader.py

@@ -18,7 +18,7 @@ _modules: dict[str, object] = {}
 
 for _name in KERNEL_NAMES:
     try:
-        _mod = importlib.import_module(f".{_name}", package=__package__)
+        _mod = importlib.import_module(f".lib.{_name}", package=__package__)
         _available[_name] = True
         _modules[_name] = _mod
     except ImportError: